HUD systems are currently used in aircraft to provide pilots with essential information superimposed onto their forward field of view through the aircraft windshield. The information displayed is typically data or symbolic images indicative of flight conditions, such as the operating condition of the aircraft, environmental information, or guidance information. HUD systems are also being designed for use in automobiles and other vehicles.
Conventional single-color HUDs include a single-color light source, a monochromatic relay lens to project the light propagating from the single-color light source, and a collimator with a combiner to provide a collimated single-color final image to the viewer. When multi-colored light passes through the multiple optical lenses of the monochromatic relay lens, the different wavelengths of light are refracted to propagate along different paths to different points of focus, resulting in an unfocused image. Apochromatic relay lenses (i.e., relay lenses implemented with many lens elements to achieve color correction by dispersion compensation), which bring different wavelengths of light to the same focus, are prohibitively expensive to manufacture and are, therefore, impractical for use in HUD systems.
A completely focused multi-colored image display would have many advantages over a single-color display. Symbols or other information in different colors could be used in a HUD system to provide different degrees of emphasis for the information presented. A color change could be used as an emergency or warning message, an enunciator, or an indicator of, for example, mode changes or out-of-tolerance conditions. A color change could also be used to indicate that an input parameter is being updated, or that immediate action is required by the pilot.
U.S. Pat. No. 5,710,668 for MULTI-COLOR HEAD-UP DISPLAY SYSTEM, which is assigned to the assignee of this patent application, describes a HUD system in which a multi-powered reflective combiner, together with a monochromatic or partly color-corrected relay lens, longitudinally corrects a multi-colored image. The combiner includes at least two curved surfaces of different optical powers. The curved surfaces carry light reflective coatings that reflect different specific wavelength ranges of colored light used in the system and transmit all other wavelengths. The curvature of each coated combiner surface is dictated by the aberrations in the color image created by the relay lens. Specifically, each coated combiner surface is positioned so that its focus coincides with the focal point of the corresponding color component in an intermediate image created by the relay lens so that the final virtual image reflected by the combiner is longitudinally color corrected. One drawback is that the light reflective coatings in a color-correcting combiner provide somewhat diminished visual transparency and clarity as compared with that provided by a conventional combiner.